This invention relates to a method of assembling electron guns for cathode ray tubes and particularly to a method of setting the cathode to G1 grid spacing.
Electron gun assemblies for cathode ray tubes of the type used with television receivers comprise one or more electron guns each of which comprises a plurality of spaced-apart, axially aligned grid electrodes and a cathode. The cathode has electron emissive material on a surface thereof for providing a beam of electrons and the grid electrodes, usually designated G1, G2, etc., perform the functions of controlling, forming, focusing and accelerating the beam. The various electrodes are typically maintained in position by radially disposed lugs which project therefrom and are fused into glass insulating rods.
The cathode to G1 grid spacing is a critical one in that it determines the cut-off voltage of the tube. It is necessary, therefore, to the scheme of mass production and interchangeability of these tubes, that close tolerances be maintained in regard to this spacing and the resultant cut-off voltage dependent thereon.
There are several methods of controlling this spacing currently in use. One of these prior methods utilizes a cup-shaped control grid which has mounted therein a cathode assembly. This assembly usually comprises a substantially cylindrical cathode sleeve, closed at one end, which is mounted in a ceramic disc. The closed end of a cathode sleeve has thereon an emissive material and it is located at a known distance from an end surface of the disc. A cylindrical spacer is affixed to the inside of the control grid and the cathode assembly is positioned within the grid so that the spacer abuts the ceramic disc. A retainer ring is usually positioned against the opposite surface of the ceramic disc and welded to the control grid. In actual use, spacing tolerances are difficult to maintain with this method. The number of pieces involved and the number of placement operations allows a buildup of error that frequently results in a rejected tube.
Another method employs an air comparator and utilizes a ceramic disc having a cathode receiving eyelet affixed thereto. The disc and eyelet are positioned in the control grid and mounted therein by any suitable means. The air comparator is then coupled to the closed end of the grid cup by suitable means, such as an O-ring seal, and a quantity of air is forced through the beam aperture therein. A cathode sleeve is inserted into the eyelet and advanced toward the closed end of the grid cup until the air flow between the grid and cathode reaches a given level, as indicated by a pressure meter, which indicates a desired spacing. Thereupon, the cathode is welded to the eyelet. There are several disadvantages to this method. It is time consuming and therefore expensive and it necessitates the cathode being assembled within the control grid before the gun is assembled, a requirement that is often desirable to avoid.
A third method utilized is one similar to that described above but instead of measuring air flow, the capacitance between the screen or G2 grid and the cathode is measured. This method required expensive and delicate measuring apparatus and expensive shielding since the cathode to screen grid capacitance is of the order of a few femto (10.sup.-.sup.15) farads. All of these prior art methods are fairly complex and time consuming and require relatively sophisticated and expensive equipment.